In recent years, a display device having an EL element as a self-luminous type light-emitting element is under intense study. In particular, a display device using an organic material as an EL material attracts an attention. The display device is also referred to as an EL display. The display device has no limitation in viewing angle because it is a self-luminous type, which is different from a liquid crystal display device.
An EL element has a structure in which a layer containing an organic compound (EL layer) is sandwiched with a pair of electrodes. The EL layer normally has a laminated structure: a structure in which a hole injecting layer/a hole transporting layer/a light-emitting layer/an electron transporting layer are sequentially laminated over an anode, or a structure in which a hole injecting layer/a hole transporting layer/a light-emitting layer/an electron transporting layer/an electron injecting layer are sequentially laminated over an anode. Fluorescent pigments or the like may be doped into a light-emitting layer. For forming these layers, either a low molecular weight material or a high molecular weight material may be used.
In this specification, a layer containing an organic compound (EL layer) is a generic name for all layers formed between a cathode and an anode. Therefore, each of the above-mentioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer is included in the EL layer. It is known that a film formation method for these organic compounds is a vapor deposition method, a spin coating method, an ink-jet method or the like.
There are two methods of forming an EL element, that is, one is a method of forming an EL layer between two types of stripe shape electrodes that are formed in such a way that they can intersect with each other (simple matrix method), and the other is a method of forming an EL layer between an opposing electrode and a pixel electrode that is arranged in matrix form and connected to thin film transistors (hereinafter, TFT) (active matrix method). However, when a pixel density increases, it is considered advantageous to use the active matrix type in which a switch is provided for every pixel (or every dot) since it can be driven at low voltage.
An organic EL element deteriorates mainly due to moisture or oxygen, thereby generating partial lower luminance or non-light emitting region. As a deterioration pattern, there is a deterioration of enlarging a non-light emitting region with time and ending up wholly generating the non-light emitting region in storing of not-driving or driving. A non-light emitting region tends to generate from the periphery of a light-emitting region. A non-light emitting region is magnified and the light emitting region appears to shrink, thus this deterioration mode is referred to as “shrinkage”. Note that FIG. 4(A) shows a light-emitting state immediately after an EL element is manufactured, and FIG. 4(B) shows a state in which shrinkage generates with time just after the EL element manufactured.
Particularly, in the case where the area of a minimum display unit (pixel) of a light emitting region is small like an active matrix EL element, it becomes dark in a whole pixel as soon as a shrink generates inside the pixel. Accordingly, it is difficult to obtain a display having a high precision (pixel pitch is small) and a high reliability when shrinkage generates, in the case of a display device using an organic EL element.